SUN SIMULATION AT HIGH FLUX LEVEL

Paul-Eric DUPUIS, Thibault RAVILY, Emmanuel SARAIVA , David LAFFONT, Nicolas GROTTO, Fabien CABARET (1)

(1) INTESPACE, 2 rond point Pierre Guillaumat, BP 64356,31029 Toulouse Cedex 4, France, Email: Paul-Eric.Dupuis@intespace.fr, Thibault.Ravily@intespace.fr, Emmanuel.Saraiva@intespace.fr,

David.Laffont@intespace.fr, Nicolas.Grotto@intespace.fr, Fabien.Cabaret@intespace.fr

ABSTRACT

SIMDIA is the first Thermal Vacuum Chamber ever built in Europe. It has been designed in the 60’s together with a sun simulator able to achieve 1400 W/m2 (1 Solar Constant) on 1 m diameter. Due to the currently on going huge ESA projects Bepi Colombo (towards Mercury) and Solar Orbiter, they are several requests to perform sun simulation tests at 10 and even 13 SC (Solar Constant) with a +/- 10% uniformity. Within a few months, Intespace has developed this year inside SIMDIA chamber the capability to achieve the requested flux levels with the right homogeneity and a very high stability. The project was challenging from several points of view and it was quite some work to define the solution, check the feasibility, implement all the modifications within a dedicated and rather short time frame in parallel with a high workload at spacecraft thermal vacuum test level in which all the thermal vacuum chambers of the company were involved during the summer period. The project was successfully managed resulting in high flux sun simulation availability by the end of September 2013 with absolutely no delay with respect to the initial planning. 1. INTRODUCTION

The two ESA projects Bepi Colombo (mission to Mercury) and Solar Orbiter have started some years ago. The main characteristic of these two huge projects is their great complexity linked to the fact that the two spacecrafts will be sent close to the sun and then submitted to very high sun flux level. The flux intensity will be in the order of magnitude of ten solar constants (SC), i.e. ten times the flux when orbiting the earth. Some instruments will even have to face thirteen solar constants. In order to check the ability of the spacecrafts and the instruments to resist to such high radiations, a new generation of sun simulators have been developed or upgraded by ESA and the European industry [1] to [3]. One of the great challenges of these solutions was to get a high flux with a good homogeneity of +/- 10%. Considering the results got by the existing small test facilities [2] & [3], INTESPACE has decided to

refurbish its old sun simulator for equipment, which was characterized by its nice homogeneity, and modify it in order to try to get the very flux intensity required. This paper summarizes the way the solution was implemented but also the different challenges which were faced by the project team. 2. Sun Simulator Modification

2.1 SIMDIA description

The thermal-vacuum test facility SIMDIA (Fig. 1) is the Intespace medium chamber and the first TVAC built in Europe (1963). Its name is coming from SIMulateur DIAmant, where Diamant was the name of the small French launcher in the 60’s. Despite this fact, it has been maintained and upgraded regularly during the last 50 years and is very well loaded as it is perfectly sized for equipment, sub-systems and mini-satellites (Fig. 2). Initially assembled in Bretigny (close to Paris), the chamber was moved to Toulouse following CNES (French space agency) in 1968 together with its sun simulator. At that time, the specifications for sun simulation was to get 1 SC on 1 m diameter with +/-7% homogeneity.

Figure 1 - SIMDIA Test facility and its sun simulator in

standard configuration

The sun simulator is made of 7 optical blocks set in the upper part of the tower and converging on a mirror at 45° to send a conical beam with 8° divergence half angle inside the chamber through a 300 mm diameter window.

Auxilliary Chamber

Main Chamber

Figure 2 - Antenna Vacuum Test preparation in SIMDIA (Sun Simulator on the left)

2.2 Specifications, hardware selection and installation

The requirements for Bepi Colombo and Solar Orbiter equipment in terms of sun illumination is to have up to 13 SC on, at least, a 200 mm diameter with +/- 10% homogeneity and a high flux stability. The first step, looking at the SIMDIA sun beam configuration (Fig. 1), was to check whether the sun intensity at 1 SC in the middle of the chamber on 1 m diameter could provide 10 SC closer to the window, taking the opportunity of the conical beam. Based on the technical sheet of the sun simulator a quick computation showed that 10 SC could be achieved on 380 mm diameter and even 13 SC on 340 mm diameter. However, the sun was very rarely switched on during the last fifteen years and only for short duration. So the second step was to modify the simulator in order to be able to get a very stable light, which was not possible with very old hardware. Basically, the target was to select the hardware (lamps, cabling, power supplies) in order to be able to run for more than 800 hours at 80% of full power with less than 3% variation per day. As two of the seven optical blocks were damaged, it was decided to renew and clean-up the five last ones. USHIO 6.5 kW Xenon lamps were selected, because well-known at Intespace for their high stability and reliability, as this type of lamps is used on a regular basis on the sun simulator of the SIMLES chamber

dedicated to big spacecraft (36 lamps to get 1 SC on 3.8 m diameter). The selection of the power supplies and cabling was the critical point to ensure the high quality of the assembly. After a first selection, series of tests were performed in order to choose the best candidate. It took a month to remove the old hardware (power supplies, cabling, water cooling device ...), install the new one and the security lines following up-to-date standards.

2.3 Sun intensity measurement at 10 SC

Performing sun simulation at 10 SC raises a lot of technical problems. One of the most important is the fact that not so many devices are able to withstand such intensity. In addition, if by chance you have one, calibrating this equipment at 10 SC to ensure, you are really measuring this level, is not an easy task. Intespace has a KENDALL Mk.IV radiometer used for years for sun intensity measurement under vacuum. It is calibrated at ambient by comparison with a PMO-6 radiometer which is calibrated à 1 SC by an external certified body. The KENDALL radiometer is the only one which can be used under vacuum. On top, it is also able to measure up to 40,000 W/m2 (i.e. 29 SC) for wavelength between 0.1 and 10 µm while the PMO-6 is measuring only up to 2,500 W/m2 (i.e. 1.8 SC) on the same bandwidth. To check the sun intensity, homogeneity and stability of the beam, it was decided to use the KENDALL for a

detailed cartography, instead of using an infra-red camera with a reference point. This allows very precise measurements while the infra-red camera measurement is linked to the emissivity of the screen on which the beam is projected, then also linked to the temperature of the screen which has then to be closely controlled. Thanks to the availability and motivation of the Intespace maintenance team, it was possible to refurbish completely an old cartographer system to which the KENDALL radiometer was attached. To give an idea of the work, some parts were missing and had to be re-invented and manufactured. The control system was perfectly obsolete (30 years old) and had to be entirely rebuilt and programmed. However, after two months of efforts, the system was working very well and able to take measurement every 5 cm on both vertical and horizontal axis. In order to allow measurement inside the narrow auxiliary chamber (located in the door of the chamber) where high intensity was expected, the radiometer was installed at the extremity of a stiff horizontal aluminium beam clamped onto the cartographer (Fig. 3).

Figure 3 – KENDALL radiometer attached to the cartographer system

To ensure that the hoses of the radiometer cooling system are not damaged by the radiations, an aluminium plate was installed in the front while a standard MLI was set to protect the vertical part of the system which is about 1 m away. At this position, not more than 2 SC was expected as the intensity is decreasing proportionally to the increase of the square of the diameter illuminated. 3. Sun Simulator Technical Acceptance

The acceptance plan was issued to control: - The spectrum - The geometry of the beam - The cartography of the sun intensity - The stability of the flux

The spectrum of the Xenon lamp was checked first (Fig. 4) and found, as expected, with some more infrared content than the solar spectrum. Inside each of the five optical blocks, there is the possibility to add an infrared filter to get the spectrum closer to the AM0 spectrum, however this option was no selected first in order to avoid an attenuation of the flux intensity. In case of need, this option is still available for the future.

Figure 4 – SIMDIA Spectrum (% relative Energy vs

wavelength in nm) The geometry of the beam was simply get by moving a white metallic panel at different distance from the sun simulator (Fig. 5). The image of the sun was then measured at low intensity giving the different diameters. From this, the divergence half angle was measured to be close to 11° for the total illumination. The useful part (+/- 10% homogeneity) was expected to be around 8° and has to be checked during the next step: the maps.

Figure 5 – Beam geometry measurements From the window, the first meter is inside the door i.e. the small auxiliary chamber whose aperture is seen in Fig. 3. Starting from the door, the main chamber is 3 m length. The middle of the main chamber is then at 2.5 m from the window. It was decided to perform a map every meter in the main chamber plus one in the middle of the auxiliary chamber. The results are gathered in table 1 below.

Location Main Chamber

Aux.

Chamb. Diameter (mm)

974

698

424

300

Flux (SC)

1.1

2.4

7.5

13.2

Flux (W/m2) 1,587 3,319 10,229 18,053

Table 1 – Beam geometry and Flux Intensity

measurements A typical example of the homogeneity get is shown in Fig. 6. The scale was selected in the following way: the peak value being 10% more than the average (if the target is to get +/- 10%), the maximum of the scale is set to 2 times the average value.

Figure 6 – Flux homogeneity at 1.1 SC

(Diameter 974 mm) The inner circle gives the part where +/-10% homogeneity is reached, while the outer circle shows the total illumination. The last one corresponds to a +/-11° divergence half angle, while the first one is +/-7.8° very closed to the 8° expected. The cartography inside the small auxiliary chamber was no so easy to perform as the protecting aluminium plate was interfering with the shrouds and blocking the motion even far from the edges. It was not possible then to measure the outer circle. Results are shown in Fig. 7.

Figure 7 – Flux homogeneity at 13.2 SC

(Diameter 300 mm) Last step was to check the stability of the flux in order to know at least the reproducibility of the cartography as it is more important to know the exact value of the intensity on a defined location than to have a variable intensity even very high. Several cartographies were then performed again on a random set of points to get the intensity at the right same location under the same sun configuration but at different times. For example, some tests were performed again one day later, after switching off and on the sun simulator. Others were performed by changing the current intensity of the lamps. To summarize, the results were very satisfying as the maximum difference between two cartographies (on a set of 40 points in average) was 2.5% (3 sigma). Moreover it was also clearly shown that the individual lamp flux can be added in a linear way to get the total sun intensity. This was a good way to check the calibration of the KENDALL radiometer at 13 SC. Indeed, as it is basically calibrated at 1 SC by comparison with the PMO-6 radiometer, the measurement is performed for each individual lamp and then for the five together at full power. The final measurement is then compared to the sum of the individual ones and the difference was never exceeding 2%, far below five times the basic calibration uncertainty. 4. Sun Simulator Operational Acceptance

In order to prepare future test campaign and thanks to the CNES collaboration, it was possible to get a realistic test specimen for the operational acceptance. A dummy of the RPW antenna of Solar Orbiter project was installed on a copper baseplate thermally controlled by a LAUDA bath (Fig. 8). The location for 10 SC was computed to be at the extremity of the auxiliary chamber.

Figure 8 – Solar Orbiter dummy RPW in the solar flux for a 10 SC test

The test was successful. Cartography was performed before and after the test with quite no difference. The 10 SC were reached with the right +/-10% homogeneity on a 380 mm diameter (Fig. 9). During the test, the temperature of the baseplate was perfectly controlled at 50°C (Fig. 10).

Figure 9 – Flux homogeneity at 10.1 SC

(Diameter 380 mm)

Figure 10 – Solar Orbiter dummy RPW test – Baseplate

temperature during the test

The sun simulator was then operational by end 2013. It has to be noticed that the SIMDIA can be operated using GN2 and LN2, which means that you can start a Thermal-Vacuum test right after a Thermal-Balance one without opening the chamber. 5. Sun Simulator evolution

Like seen in Fig. 8, the 10 SC are reached at the extremity of the small auxiliary chamber which may imply some difficulties in case of specimen greater than 400 mm diameter (the auxiliary chamber is a bit more than 600 mm diameter). In order to keep the advantage of the very good sun homogeneity and stability and remove the drawback of the conical beam and narrow chamber, it was decided by end 2013, to install and additional lens in order to focus more the beam and get a nearly parallel beam. A cheap solution, to check the feasibility, was to use an existing spare lens either from the SIMLES (L3B is 540 mm diameter and could be installed inside the auxiliary chamber) or from the metrology bench (M1 lens is 300 mm diameter). In order to minimize the complexity of the modification, the M1 lens was selected and installed outside the chamber (Fig. 11 – in green).

Figure 11 - SIMDIA Test facility and its sun simulator with the correction to get a parallel beam

The provisional installation gives already very good results as the beam obtained is now nearly parallel with only +/- 1° divergence half angle. The point is that, as the lens has to be set a bit too far from the sun simulator to get the right convergence, a part of the light is lost being out of the converging lens (Fig. 12a). The 10 SC are then achieved on a smaller diameter than expected (240 mm instead of 380 mm) but at least in the main chamber. A study has now started to define the optimal lens to be installed to achieve the same results obtained with the conical beam inside the auxiliary chamber, but with a parallel beam inside the main chamber. The results are expected by the middle of the year.

In between, a quick improvement could be to install a reverberant tube between the sun simulator and the converging lens to collect additional light (Fig. 12b).

Figure 12 – Effect of the reverberant tube on SIMDIA

sun simulator optical path with the additional converging lens

6. CONCLUSION

Within a few months, Intespace has developed in 2013 inside SIMDIA thermal-vacuum chamber the capability to achieve very high flux levels with the right homogeneity and a very high stability. The project was challenging from several points of view and it was quite some work to define the solution, check the feasibility and implement all the modifications within a dedicated and rather short time frame. On top, this was done in parallel with a high workload at spacecraft thermal vacuum test level in which all the thermal vacuum chambers of the company were involved during the summer period. The project was successfully managed resulting in sun simulation availability at 10, 13 and even more solar constants with +/-8° half angle divergence, +/- 10% homogeneity and very high stability. The feasibility to get the beam quasi parallel and the high flux inside the main chamber has been successfully performed and this solution will be implemented in a permanent way by the middle of this year.

7. REFERENCES

[1] Alexandre Popovitch, Rene Messing, Andre Tavares, Steven Sablerolle – Upgrade of ESA Large Space Simulator for providing Mercury environment – 26th Aerospace Testing Seminar – Los Angeles, USA – 28-30 March 2011.

[2] N Thomas, T Beck, S Chakraborty, M Gerber, S Graf, D Piazza, G Roethlisberger - A wide-beam continuous solar simulator for simulating the solar flux at the orbit of Mercury – IOP Measurement Science and Technology, 22 (2011) 065903 (11pp) – 13 May 2011.

[3] Page J., Popovitch A., Wagner M., O'Neil S., Appolloni M., Casarosa G. - ESA's Small Thermal Vacuum Test Facility with High Flux Simulation Capability VTC1.5 - 12th European Conference on Spacecraft Structures, Materials & Environmental Testing – Noordwijk, The Netherlands – 20-23 March 2012.

Sun Simulator

Reverberant Tube

Converging Lens

SIMDIA

Lamps

SIMDIA

Lamps

Sun Simulator

Converging Lens